In known electronic matrix systems, an array of storage elements, each having a unique address, is utilized for storing electric charge and can include, for example, memory arrays and/or LCDs. In LCDs, the storage elements take the form of picture elements or pixels. The pixels generally include a pair of spaced apart electrodes having liquid crystal material disposed therebetween. Thus, each pixel constitutes a capacitor in which electric charge can be stored. The charge stored in a pixel results in a voltage potential across the opposing electrodes and an electric field across the liquid crystal material. By controlling the amount of charge stored in pixels across the array, the properties of the liquid crystal (LC) material can be controlled to obtain a desired light influencing effect or image which is displayed to a viewer.
In LCDs, alignment of the LC molecules can be obtained when the electric field applied to the LC is above a threshold value. When this occurs, a pixel becomes light transmissive or light absorbing, depending upon the relative alignment of the display's polarizers and alignment layers; when the field is below the threshold value, an opposite effect is obtained.
In LCDs, it is necessary to update the condition of each pixel at regular intervals, i.e. at a given frame rate. This is required because the pixels can retain or store the applied charge potentials for only a finite time. Updating is further required in order to change the image to be displayed (e.g. when the image is changing or moving). Accordingly, the ability to rapidly transfer to, and store electric charge in, pixels and to efficiently retain the stored charge therein for a frame period is essential.
U.S. Pat. No. 4,731,610 (to co-inventor Baron herein) discloses a driving scheme which utilizes, for example, either field effect transistors (FETs) or PIN (p doped-intrinsic-n doped) diodes. Unfortunately, these types of switching devices (PIN diodes and FETs), when employed in LCDs, require an undesirably high number of mask steps for fabrication, and also require a high degree of critical alignment. Accordingly, these complex structures reduce the yield of usable components per fabrication run, and therefore increase production costs.
Metal-insulator-metal (MIM) diode LCDs are easier to fabricate than FET/TFT LCDs and conventional diode LCDs. A typical MIM electronic matrix array requires between two and four thin film layers and photomask steps, as compared to 6-9 thin film layers and photomask steps for TFT arrays. Patterning of most MIM arrays can be achieved with less stringent overlay accuracy and resolution requirements, then is required for TFT arrays. As a result, less expensive photo-exposure equipment, such as scanning projection aligners, can be used, that have more than twice the throughput and cost less than half as much as flat panel steppers.
Despite their lower production costs, MIM driven LCDs are not widely used. This can be attributed to the inferior performance of typical MIM LCDs with regard to gray shade control, image retention, response time, and maximum size and resolution as compared to TFT LCDs. Accordingly, there exists a need in the art for an improved MIM LCD drive scheme, which is cheaper to manufacture, less susceptible to image retention and gray scale problems, and has good resolution.
European Patent Application 0 434 627 A2 discloses a MIM diode driven LCD, invented previously by Inventor Willem den Boer of the instant application. In the EP '627 application, the MIM diode driven LCD includes two select lines per row, and a corresponding column line for each column. Thus, each individual pixel is addressed by a single column line and a pair of select or row lines. First and second MIM diodes are utilized to address each pixel, with a one MIM per branch design being utilized (i.e. there is only a single MIM diode between the common node and each select line). Unfortunately, the drive scheme of the '627 application suffers from at least the following problems. Firstly, as shown in FIG. 3 of the '627 application, the polarity of signal on each select line is always reversed, with the reversal interval being two frames. Thus, for example, the pulses on a given select line will be positive for two frames, and then switch to a negative polarity for the next two frames, and so on. This requires sophisticated and expensive electronics which are undesirable. Secondly, the single MIM per branch design in the '627 application can result in a non-symmetric device, which requires the polarity changes discussed above. Thirdly, as shown in FIG. 3(a) of the '627 application, for example, a large number of separate voltage levels (or holding voltage levels) are required in this design. This also is expensive, and requires complex circuitry which is undesirable.
Fourthly, as shown in FIG. 3(a) of the '627 application, for example, the voltage swing is 34 volts. This is so large, so as to require complex and expensive drive circuitry.
A fundamental desire in LCD driving schemes, is to reduce the complexity and costs of drive circuitry, which is always a cost sensitive component to be taken into consideration.
U.S. Pat. No. 5,576,728 to Maeda discloses a dual MIM circuit. Unfortunately, the MIM circuit of the '728 patent relate to "frame" inversion (e.g. see FIG. 7 herein). Frame inversion is undesirable in that it leads to top-to-bottom shading, cross-talk, and flicker in display operation and is therefor almost never used for display applications. Row inversion or pixel inversion are instead typically used in LCD applications.
In the '728 patent, a reduction of pixel voltage variation across the display (shading) from 0.5 V to 0.1 V is described as a benefit of the proposed '728 patent drive scheme. The reduction is allegedly achieved because the total swing of the data voltage and therefore cross talk is reduced by using an offset voltage on the select or non-select voltage. This allows the use of a low voltage column driver with bipolar video.
It has been found by the instant inventors that the shading described in the Maeda '728 patent can only be completely eliminated by row inversion or pixel inversion (which are much different than Maeda's frame inversion), in which cross talk from the data voltage is substantially canceled out for most image patterns. In Maeda, the select voltages on positive and negative select lines are kept constant for all pixels on all rows during one scan interval. Also in Maeda, the bipolar video occurs around a non-zero offset voltage that stays constant during the entire scan interval, and this can only be done for frame inversion. Other schemes in Maeda '728 are also only compatible with frame inversion and are therefore useless in all practicality. Some other Maeda schemes use an offset on the non-select bias. However, because this offset is the same for all rows and changes polarity at the end of the scan interval, it is not a true holding voltage that is continuously maintained until the same row is selected again. The offset is thus only compatible with frame inversion and will continue to cause shading and flicker problems. Thus, Maeda is incompatible with desirable row and pixel inversions.
It is apparent from the above that there exists a need in the art for an improved MIM diode driven LCD or other electronic matrix array, which (i) is designed so as to require less complex and less expensive circuitry; (ii) has improved gray shade control; (iii) is less sensitive to image retention than previous MIM LCDs and has had good resolution characteristics; and (iv) is capable of being used with row inversion systems that are commonly used in LCDs and the like.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.